Rocks that are exposed to the high pressure and temperatures associated with mountain building are said to have undergone regional metamorphism. According to Columbia University, regional metamorphism takes place under mountain chains, at the collision point between continental plates and in other high-pressure, high-temperature zones.
Regional metamorphism slowly changes the crystal structure of rocks over periods of thousands or millions of years. The crystals are reconfigured while remaining in their solid state, according to Columbia University, and so require large inputs of energy to make the transformation. The bulk chemical composition of the rock is more or less unchanged from the original rock. Deep burial in a mountain-building, or orogenic, belt causes the rock's crystals to reorient perpendicular to the line of stress and reconfigure into structures that are stable at the high temperatures and pressures associated with burial under an orogenic region. Mica and horneblend are two platy minerals that are especially prone to this recrystalization process.
When the minerals reorient in lines parallel to the principal applied stress, the result is known as foliation. According to Columbia University, slate, schist and gneiss are some of the most common foliated rock types found in these reconfigured deposits. Slate is commonly the result of low-grade metamorphism, while schist and gneiss require higher temperatures and pressures.